The dark brown specimens labeled as coming from “Mouth Río Inuya, Río Urubamba” and “Boca Río Urubamba”, collected by the Olalla brothers and presently in the collection of the American Museum of Natural History, were identified by Hershkovitz (1990) as belonging to Callicebus brunneus. However, after observation in the wild, a study of museum specimens and an analysis of the literature, we propose that these animals represent an undescribed species.

Diagnostic characters: Forehead with a jet-black band extending to behind the ears. Ears black, covered with long black hairs. Hairs of the cheeks brownish-agouti with long black tips, directed forwards and giving the cheeks, from a distance, a black color. Chin brown agouti. Facial skin black with black hairs on the cheeks and many white hairs on the nose and around the mouth. Pupils black and irises light brown. Dorsal and lateral side of the body, including the upper arms and the legs, brownish-agouti. Crown the same color as the back. Hands and inner side of the forearms black. Outer side of the forearms black up to the elbow, mixed with a small amount of agouti hairs. Feet black, knees darker than the rest of the leg, giving them a blackish hue. Lateral side of the body, inner side of the upper arms and legs brown-agouti,
paler colored than the back. Basal half of the tail almost black, mixed with some brown agouti hairs, becoming lighter towards the end, with a greyish tip. Paratype 15912 differs in having the forearms less
black, with more brown-agouti hairs and a few lighter colored hairs on hands and feet. The same variation is also seen in the six AMNH specimens from the Río Urubamba. We did not observe any variation in the 35 individuals that we see the details of these dark monkeys in a dark forest.

Comparisons: Distinguished from the partially sympatric Callicebus toppini by its dark color and brownish underparts. Along the Río Tambo it is probably allopatric with Callicebus discolor from which it is easily distinguished by its black forehead and brown color. It is distinguished from the BrazilianCallicebus brunneus (Wagner) by the coloration of the head. In C. urubambensis, the occiput and the sides of the face are the same brown color as the back. In C. brunneus, there is a dark brown band behind the jet-black frontal blaze, separating it from the yellowish occiput. The yellowish coloration extends towards the neck, where it becomes the same agouti-brown color as the back and the sides of the body. The cheeks of C. brunneus are dark brown, conspicuously darker than the sides of the body. There is some variation in the coloration of the lecto-(para)-types of C. brunneus, but none has the black forearms
of C. urubambensis. There is a geographical gap of more than 600 km between the most eastern observation of C. urubambensis and the most western confirmed sighting of C. brunneus. Genetic studies could elucidate the taxonomic relationship between the two species.

Etymology: This species is named after the Río Urubamba, Peru, where it was discovered.

Vernacular name: The species is locally known as “mono tocón.” We propose the name Urubamba brown titi monkey.

Geographical distribution: We encountered C. urubambensis at 12 localities on the left bank of the Río Urubamba. We did not observe it on the right bank of that river or on the left bank of the Río Tambo. The paratypes came from the Quebrada Calientes, on the left bank of the upper Río Madre de Dios, on the eastern border of Manu National Park (71°16’8″W, 12°40’50″S). A photograph of C. urubambensis
was taken near the Amazonia Lodge, also on the left bank of the upper Río Madre de Dios (71°22’10″W, 12°51’57″S) (Figs. 33−35), while the species was also observed further downriver, near Pantiacolla Lodge (71°14’31″W, 12°39’36″S) and Yine Lodge (approx. 70°55’45″W, 12°16’03″S) (J. Vermeer, pers. obs.). Despite the presence of many researchers and tourists in the Tambopata Nature Reserve and the Los
Amigos Biological Station further to the east, there is no evidence that Callicebus urubambensis occurs east of the upper that the titi monkey in those areas is the generally misidentified C. toppini.
Although we know that rivers are not absolute geographical barriers for titi monkeys, especially in areas where rivers constantly change their course, we used large rivers to indicate the distributions of the species. We estimate that the range of Callicebus urubambensis includes the lowland forest area between the right bank of the Río Tambo and the left bank of the Río Urubamba, and the lowland forest
between the left bank of the Río Manu and the left bank of the upper Río Madre de Dios. The species’ distribution is further restricted by the presence of mountain ridges to the west and south. The situation near the upper Río Urubamba needs further investigation. On both sides of the Río Camisea there
are confirmed records only of C. toppini (T. Gregory, pers. comm.). However, somewhere there, there must be a connection between the western and eastern part of the distributions of C. urubambensis, unless in recent history the species has been replaced in that area by C. toppini. The species has been
observed to live in sympatry with C. toppini in the eastern part of its distribution, on the left bank of the upper Río Madre de Dios (J. Vermeer, pers. obs.).

Systematics: Hershkovitz (1990) and Kobayashi (1995) divided the titi monkeys into species groups. Considering the resemblance in coloration, we would be tempted to place C. urubambensis in the same group as C. brunneus, which Hershkovitz (1990), Kobayashi, (1995) and Van Roosmalen et al. (2002) have in the moloch group. Van Roosmalen and Van Roosmalen (2014), on the other hand, placed it in the cupreus group. Considering, however, the coloration of neighboring species, we propose that it aligns with the donacophilus group (Kobayashi 1995). Following the “metachromism bleaching theory” (Hershkovitz 1988; Van Roosmalen and Van Roosmalen 2014), C. urubambensis would be close to the
archetypical taxon of this species group. The dark forehead, forearms, hands and feet show that the species underwent considerable eumelanin saturation, but the process switched to pheomelanin bleaching when members of the species group radiated northwards (C. oenanthe, which itself shows pheomelanin bleaching northwards in its restricted range) and south-eastwards (C. modestus to C. olallae to C. donacophilus to C. pallescens).

Conservation: The Urubamba brown titi is hunted for food, especially where all the larger primates have been exterminated. As it lives near villages, it is an easy prey for hunters and young boys with slingshots. However, considering its relatively large range with low human presence, there is no immediate threat for this species. It is protected in Manu National Park, and is common along the Río Urubamba (see
also Aquino et al. 2013).